McIntosh MC 2250 User manual

THEMclNTOSHMC2250SOLIDSTATESTEREOPOWERAMPLIFIER
ReadingTime:31Minutes Price$2.00

VARIOUSREGULATORYAGENCIESREQUIRETHATWEBRINGTHEFOLLOWING
INFORMATIONTOYOURATTENTION.PLEASEREADITCAREFULLY.
WARNING:TOPREVENTFIREORSHOCK
HAZARD,DONOTEXPOSETHISUNITTO
RAINORMOISTURE.
TheMclntoshyouhavepurchasedisa Model
MC2250.Ithasa serialnumberlocatedontherearpanel
ofthechassis.Recordthatserialnumberhere:
Themodel,serialnumberandpurchasedateareim-
portanttoyouforanyfutureservice.Recordthepur-
chasedatehere:
Uponapplication,MclntoshLaboratoryprovidesa
Three-YearServiceContract.YourMclntoshAuthorized
ServiceAgencycanexpediterepairswhenyouprovide
theServiceContractwiththeinstrumentforrepair.To
assist,recordyourServiceContractnumberhere:
SerialNumber
Purchasedate
ServiceContractNumber

YourMC2250StereoPowerAmplifieris
designedtoperformtoitsspecificationsfor
manyyears.Ifyouhaveanyquestions,please
contact:
CUSTOMERSERVICE
MclntoshLaboratoryInc.
2ChambersStreet
Binghamton,NewYork13903-9990
Phone:[phone]
MclNTOSHTHREEYEARSERVICECONTRACT
AnapplicationforA THREEYEARSERVICECONTRACTisincludedwiththismanual.
Thetermsofthecontractare:
1.Mclntoshwillprovideallparts,materials
andlaborneededtoreturnthemeasured
performanceoftheinstrumenttothe
originalperformancelimits.TheSER-
VICECONTRACTdoesnotcoverany
shippingcoststoandfromtheauthoriz-
edserviceagencyorthefactory.
2.AnyMclntoshauthorizedserviceagency
willrepairMclntoshinstrumentsatnor-
malservicerates.Toreceiveservice
underthetermsoftheSERVICECON-
TRACT,theSERVICECONTRACTCER-
TIFICATEmustbepresentedwhenthe
instrumentistakentotheserviceagency.
3.Alwayshaveservicedonebya
Mclntoshauthorizedserviceagency.If
theinstrumentismodifiedordamaged
asa resultofunauthorizedrepair,the
SERVICECONTRACTwillbecancelled.
Damagebyimproperuseormishandl-
ingisnotcoveredbytheSERVICE
CONTRACT.
4.TheSERVICECONTRACTisissuedto
youastheoriginalpurchaser.Topro-
tectyoufrommisrepresentation,this
contractcannotbetransferredtoa se-
condowner.
5.ToreceivetheSERVICECONTRACT,
yourpurchasemustbemadefroma
Mclntoshfranchiseddealer.
6.Yourcompletelyfilledinapplicationfor
theSERVICECONTRACTmustbepost-
markedwithin30daysofthedateof
purchaseoftheinstrument.
7.ToreceivetheSERVICECONTRACT,all
informationontheapplicationmustbe
filledin.TheSERVICECONTRACTwill
beissuedwhenthecompletelyfilledin
applicationisreceivedbyMclntosh
LaboratoryIncorporatedinBinghamton,
NewYork.
8.UnitsinoperationoutsidetheUnited
StatesandCanadaarenotcoveredby
theMclntoshSERVICECONTRACT,ir-
respectiveoftheplaceofpurchase.Nor
areunitsacquiredoutsidetheU.S.A.
andCanada,thepurchasersofwhich
shouldconsultwiththeirdealertoas-
certainwhat,ifany,servicecontractor
warrantymaybeavailablelocally.
Copyright1982© byMclntoshLaboratoryInc.
-1-
Contents
SERVICE1
INSTALLATION2
HOWTOCONNECT4
FRONTPANELCONTROLS
ANDHOWTOUSETHEM9
REARPANELINFORMATION11
PERFORMANCELIMITS12
PERFORMANCECHARTS13
TECHNICALDESCRIPTION15
BLOCKDIAGRAM17
TakeAdvantageof3 years
ofContractService...
FillintheApplicationNOW.

Removethethreescrewssecuringtheshipping
palletandremovethepalletandbottomshipping
pad.Removetheplasticshippingbag.Ifthe
amplifieristobemountedina cabinetorrack
mount,removethefourplasticfeetfromthebottom
oftheamplifier.Iftheamplifierisusedalone,leave
thefeetscrewedtotheamplifier.Theyare
necessarytoholdtheamplifierabovethesupporting
surfacetoallowadequateventilation.Theamplifier
can notbeusedwithoutventilationthroughthe
chassisandheatsinks.
INSTALLATIONINA MclNTOSHCABINET
Installtheamplifierbyfollowingtheinstructions
enclosedwiththecabinet.
TheMC2250canbehousedina MclntoshL54
cabinet,installedina customcabinetormountedin
astandard19inchrackbyusingtheadditionof
RACKMOUNTINGADAPTERS(RMA7).TheMC2250
weights80pounds(36.3kg).Inanyinstallationitis
necessarytoprovidepropersupportfortheweight.
Themountingmethodsdescribedandtheamplifier
havebeendesignedtoaccomodatetheweight.Be
surethatthepanelinwhichtheamplifierismounted
canalsohandletheweight.
UNPACKINGTHEAMPLIFIER
Removetheamplifierandhardwarepackagefrom
theouterandinnershippingcartons.Becarefulto
avoidmaringorplacinga strainontheedgesofthe
frontpanel.Placetheamplifierupsidedownona
surfacethatwillsupportthetransformerandheat-
sinksbutwillnottouchtheedgeofthefrontpanel.A
padfromtheshippingcartoncanbeusedforthis
purpose.
INSTALLATIONINA CUSTOMCABINET
Thetrouble-freelifeofanelectronicinstrumentis
greatlyextendedbyprovidingsufficientventilation
aroundit.Thispreventsthebuildupofhighinternal
temperaturesthatcausedeterioration.Allow
enoughclearancesothatcoolaircanenteratthe
bottomofthecabinetandbeventedfromthetop.
Withadequateventilationtheinstrumentcanbe
mountedinanyposition.Therecommended
minimumspaceforinstallationis15inches(38.1
cm)deep,17inches(43.2cm)wide,and8 inches
(20.3cm)high.
Verticalmountingoftheamplifierrequirescareful
consideration.Thetotalweightissupportedbythe
panelandshelfonwhichtheamplifieristobe
mounted.Thoughtshouldbegiventoprovidinga
meansforraisingtheamplifieroutofthepanel.
Moreconsiderationshouldbegiventoproviding
adequateventilationatthepanelinverticalmoun-
ting.Theheatsinksareattherearofthechassisper-
mittingtheheattopassovertheentirechassis.Itis
thentrappedbehindthepanelcausingtheoverall
temperaturetorise.Inanycase,airflowforcooling
canbeimprovedbytheuseofa properlyinstalled
lownoisefan.Adequatecoolingwillextendthelife
oftheamplifier.
DEPTHREQUIRED
TOPOFSHELF& BOTTOM
OFCUTOUTSHOULDCOINCIDE.
Fig.1.Viewsshowingmountingshelfinposition.
-2-
MOUNTINGSCREWS
DIFFERENCEBETWEENSCREW
LENGTH& TOTALSHELFTHICKNESS
MUSTNOTEXCEED1/2"
SHELF

1/4"DIA.(2HOLES)
Fig.2.Topviewofthepanelcutoutandmountingshelf.
INSTALLATIONPROCEDURE
1.MarkforPosition
Markthecabinetpanelwheretheinstrumentis
tobeinstalled.ThesolidlinesinFig.1 represent
theoutlineoftherectangularcutouttobemade
andalsorepresenttheoutsidedimensionsofthe
chassis.Makesuretoclearinternalshelves,parti-
tions,oranyequipment.Thebrokenlinerepre-
sentstheoutsidedimensionofthepanel.
2.DrillHoles
Usinga drillbitslightlylargerthanthetipof
yoursawblade,drilloneholeateachoftwodia-
gonallyoppositecorners.Theholesshouldbarely
touchtheinsideedgeoftheoutline.
3.SawPanelCutout
Placethebladeofthesawinoneoftheholes.
Sawingcarefullyontheinsideofthelines,first
makethetwolongcutsandthenthetwoshort
cuts.Aftertherectangularopeninghasbeencut
out,usea filetosquarethecornersandsmooth
anyirregularitiesinthecutedges.
4.ShelfInstallation
Inthesamewaymarkandsawtheventilation
cutoutshowninFig.2.Alsodrilltwo¼"holesfor
themountingscrews.Installtheshelftothe
cabinetpanel.Itmustbeabletosupportthetotal
weightoftheinstrument.
5.InstallInstrument
GuidetheACpowercordthroughthepanel
openingtothebackofthecabinet;then,slidethe
instrumentintotheopeninguntilitsfrontpanelis
flushwiththecabinetpanel.Securetheamplifier
totheshelfwith#10woodscrewsoftheproper
length.
½"thickshelf—use1"screws
¾"thickshelf—use1¼" screws
Donotuselongerscrews.Theymaypenetrate
electroniccomponentsinsidethechassisand
causeseveredamage.Thescrewlengthmustnot
exceedtheshelfthicknessbymorethan½".
INSTALLATIONINA 19INCHRACK
TheoptionalRMA7RACKMOUNTINGADAPTERS
installoneachendoftheamplifierfrontpanel.On
eachsideoftheamplifier,threescrewssecurethe
frontpanelbrackettothechassis.SeeFig.3.
Removethesethreescrews,placetheRACKMOUN-
TINGADAPTERoverthescrewholes,andreplace
thescrews.Theentireamplifierweightistransmit-
tedthroughtheRACKMOUNTINGADAPTERStothe
rack.Additionalsupportfortheamplifiercanbepro-
videdattherearofthechassisasdesired.
Fig.3.RackMountingAdaptersinstalled.
-3-
VENTILATION
CUTOUT
PANEL
CUTOUT
FRONTPANEL
MTG.SURFACE

HowtoConnect
INPUTCONNECTIONS
STEREOOPERATION
Useshieldedcablestoconnectthesignalfrom
thepreamplifierorsignalsourcetothepower
amplifier.Tominimizethepossibiltyofhumthe
shieldedcablesshouldberunparalleltoeachother
orlooselytwistedtogether.Locatethecablesaway
fromspeakerleadsandACpowercords.Allconnec-
tionsaremadeonthebackpaneloftheMC2250.
Forstereooperation,theleftoutputofthe
preamplifiershouldbepluggedintotheLeftINPUT
jackofthepoweramplifier.Therightoutputofthe
preamplifiershouldbepluggedintothe
Right/MONOINPUTjackofthepoweramplifier.
InstereooperationtheMODESWITCHmustbein
theSTEREOposition.
MONOPHONICOPERATION
FormonophonicoperationtheMODEswitchon
thebackpaneloftheMC2250mustbeplacedinone
oftheMONOpositions.Fortheproperchoice,see
thesectiononmonophonicoutputconnections.
Connecta shieldedcablefromthesignalsourceto
theRight/MONOINPUTjackoftheMC2250.The
LeftINPUTisnotused.
BecertainthattheMC2250isneveroperatedin
thestereomodewiththeoutputsconnectedfor
monophonicoperation.ShouldtheMODESWITCH
beleftintheSTEREOpositionandtheoutputsre-
mainstrappedfora monoparallelload,one
amplifierchannelwillattempttodrivetheother
whichcauseshighcirculatingcurrentsand
overheating.
OUTPUTCONNECTIONS
BecertaintheloudspeakersconnectedtotheMC
2250arecapableofhandlingthepoweroutputofthe
amplifier.
Selectionofthepropergaugewiretoconnectthe
loudspeakerstotheamplifierpreservesthequality
ofsoundreproductionforwhichtheloudspeakers
havebeendesigned.Ifundersizewireisused,
resistanceisaddedtotheamplifier/loudspeaker
combinationwhichadverselyaffectstheperfor-
mance.Addedresistancecausesdepreciationof
dampingcharacteristics,modificationoffrequency
responseandreductioninpowertotheloudspeaker.
Uselampcordorwirewithsimilarinsulationto
connectthespeakerstotheamplifier.Inallcases,
theleadstoandfromthespeakershouldbetwin
parallelconductorortwistedpair.Whenusing8
ohmspeakersandforthenormallyshortdistances
ofunder30feetbetweentheamplifierandspeaker,
#18wireorlargercanbeused.Fordistancesover30
feeta largerdiameterwireisrequired.Selectthe
correctsizefromthechartbelow.TheDCresistance
ofthespeakerleadsshouldbelessthan5%ofthe
speakerimpedance.Resistanceoftheleadsmust
becomputedforthelengthofwirebothtoandfrom
thespeakerorspeakers.
MAXIMUMWIRELENGTHS
Wire
Gauge
18
16
14
12
10
For4 OhmLoad
Feet
15
25
40
60
100
Meters
4.57
7.62
12.19
18.29
30.48
For8 OhmLoad
Feet
30
50
80
120
200
Meters
9.14
15.24
24.38
36.58
60.96
Wirelengthsaboverepresentthewireresistance
equalto5%ofthespeakerimpedance.
Formultiplespeakeroperation,runseparate
leadsfromtheamplifiertothespeakers.
STEREOOPERATION
Firstchecktheimpedanceofthespeakertobe
connected.Thisisusuallyfoundonthespeaker
systemneartheterminalsorinthespeakerlitera-
ture.TheproperterminalsontheMC2250canthen
beused.Connectthe+ orratedimpedanceter-
minaloftheleftspeakertothecorrespondingLEFT
OUTPUTterminalontheamplifier.Connectthe- or
thecommonterminaloftheleftspeakertotheleft
COMMONoutputterminaloftheamplifier.Inthe
samewayconnecttherightspeakertotheamplifier
RIGHTOUTPUTterminals.
-4-

Whenmultiplespeakersaretobeconnectedto
theamplifier,thecombinedloadimpedanceforthe
speakersmustbecalculated.Theloadmustthenbe
connectedtotheappropriateamplifierimpedance
tap.Usethistabletoaidinselectingthecorrectim-
pedancematch:
STEREONORMALCONNECTIONS
Load
impedance
inohms
0.8to1.6
1.6to3.2
3.2to6.4
6.4andup
Connectfor:
1ohmoutput
2ohmoutput
4ohmoutput
8ohmoutput
Ifa loadimpedanceisusedthatislowerthanthe
outputimpedancetap,thenreducedpowerandpos-
sibledistortionwillresult.Ifa loadimpedanceisused
thatishigherthantheoutputimpedancetap,then
neitherthesignalnortheamplifierwillbeharmed
butthepoweravailableisreduced.
STEREOCONSTANTVOLTAGELINEOPERATION:
Whenspeakersaretobeusedinmanydifferent
rooms,ormanyspeakersareneededtocovera large
areawithsound,constantvoltagelineoperationcan
beused.Thisisconvenientforlargerhomeback-
groundmusicsystemsorforindustrialinstallations.
Appropriatelinematchingtransformersmustbeused
ateachspeakerlocation.The2 ohmoutputprovides
thestandard25voltconstantvoltagelineoutput.In
thestereomode250wattsisavailableforeach
channel.
STEREOCONSTANTVOLTAGELINECONNECTIONS
Foroutput
voltageof
25volts
Connectfor:
2ohms
MONOPHONICORSINGLECHANNELOPERATION
WhentheMC2250isusedasa monophonicor
singlechannelpoweramplifierthetwochannels
canbecombinedintwodifferentways.Bothwill
combinetoproduceupto500wattsbuttheavailable
impedancesaredifferent.MONOBRIDGE offersa
fewhigherimpedanceconnectionsthanMONO
PARALLEL.Eithermodecanbeusedfor2 or4 ohm
operation.Performanceisidentical.Monophonic
constantvoltagelineoperationcanalsobeused.
Theoutputsareconnectedasdescribedbelow.
WhenusingtheMONOBRIDGE mode,outputim-
pedancesof2,4,8,and16ohmsareaccomodated.
Outputconnectionsaremadebyconnectingtothe
outputterminalsaslistedbelow.Notethatneither
outputterminalisatgroundpotential.
MONOBRIDGECONNECTIONS
LoadImpedance
InOhms
2
4
8
16
Connect- Speaker
LeadTo:
Left1 OhmTerminal
Left2 OhmTerminal
Left4 OhmTerminal
Left8 OhmTerminal
Connect+ Speaker
LeadTo:
Right1 OhmTerminal
Right2 OhmTerminal
Right4 OhmTerminal
Right8 OhmTerminal
WhenusingtheMONOPARALLELmode,output
impedancesof½,1,2,and4 ohmsareac-
comodated.Connectaslistedbelow.Thecommon
outputterminalsareatgroundpotential.
MONOPARALLELCONNECTIONS
LoadImpedance
InOhms
½
1
2
Connect- Speaker
LeadTo:
EitherLeftorRight
CommonTerminal
EitherLeftorRight
CommonTerminal
EitherLeftorRight
CommonTerminal
EitherLeftorRight
CommonTerminal
Connect+ Speaker
LeadTo:
BothLeftandRight
1OhmTerminals
BothLeftandRight
2OhmTerminals
BothLeftandRight
4OhmTerminals
BothLeftandRight
8OhmTerminals
MONOPHONICCONSTANTVOLTAGELINE
OPERATION
WhentheMC2250isusedinMONOforconstant
voltagelineoperation,theMONOPARALLELcon-
nectionisused.Theappropriatelinematching
transformersmustbeusedateachspeakerlocation.
The1 ohmconnectionprovidesthestandard25volt
constantvoltagelineoutput.Inthismodea totalof
500wattsofmonophonicpowerisavailable.
MONOCONSTANTVOLTAGELINECONNECTIONS
Foroutput
voltageof
25volts
Connectedfor:
1ohmoutput(mono),monoparallel
BecertainthattheMC2250isneveroperatedin
thestereomodewiththeoutputsconnectedfor
monophonicoperation.ShouldtheMODESWITCH
beleftintheSTEREOpositionandtheoutputsre-
mainstrappedfora monoparallelload,onechannel
willattempttodrivetheotherwhichcauseshighcir-
culatingcurrentsandoverheating.
ACPOWER
TheMC2250operateson120volts50/60Hzand
requires12ampsatfullratedoutput.Plugthepower
cordintoa suitablesource.TheauxiliaryAC
OUTLETontheMC2250isnotfusedorswitched.
-5-

Stereophonic
Connections
-6-

Mono-Bridge
Connections
-7-

Mono-Parallel
Connections
-8-

TheFrontPanelControlsand
HowtoUseThem
GAINCONTROLS
Whenyoursystemisfirstconnected,turnthe
LEFTandRIGHT/MonoGAINcontrolstominimum
(fullycounterclockwise)beforeturningthepoweron.
Thecontrolscanthenbeadvancedtothedesired
settings;normallymaximum(fullyclockwise).When
usingtheMC2250forMONO-BRIDGEorMONO-
PARALLELoperation,usetheRIGHT/MonoGAIN
control.TheLEFTchannelcontrolisnotusedinthe
MONOmodeofoperation.
HEADPHONES
ThestereoHEADPHONEjackislocatednextto
thepowerswitch.Theheadphonesaredrivenbyleft
andrightpreampsectionsintheMC2250.The
preampcircuitshavebeendesignedtopowerlow
impedancedynamicheadphones.Highimpedance
headphoneswillworkbutata reducedlistening
level.TheHEADPHONEoutputisnotaffectedbythe
SPEAKERswitch.
SPEAKERS
TheSPEAKERswitchturnstheloudspeakerson
oroff.TheswitchmustbeintheONpositiontohear
soundfromtheloudspeakers.Whentheswitchis
OFF,youcanlistentoheadphonesinprivate.
POWER
ThePOWERswitchturnstheMC2250ONorOFF.
Eachtimethepoweristurnedon,theredSYSTEM
TESTreadoutandgreenPOWERGUARDlightswill
bealternatelyvisibleastheselftestsequenceauto-
maticallyproceeds.Theswitchdoesnotcontrolthe
poweroutletonthebackpanel.Ifyouwishtocon-
troltheACpowerfroma preamplifiercontrolcenter,
plugtheACcordoftheMC2250intoa controlled
outletontherearofthepreamplifiercontrolcenter
andleavethePOWERswitchintheONposition.
POWERGUARD
EachchanneloftheMC2250isequippedwitha
POWERGUARDcircuit.POWERGUARDprevents
theMC2250fromcausinghighdistortionwhenthe
inputisoverdriven.Itactsautomaticallywhendis-
tortionexceeds0.5%.Aslongastheamplifierisnot
overdriven,theNORMALgreenPOWERGUARD
lightswillstayon.Iftheinputsignalisexcessivefor
eitherorbothchannels,thecorrespondingredLIMIT
lightwillgooninstead.Thesoundwillremainclear
andundistorted.Whentheexcessivedrivingsignal
isremoved,theNORMALgreenlightswillreturn.
-9-

SYSTEMTEST
TheMC2250containsa newautomatictestcir-
cuit.Eachtimetheamplifieristurnedon,anLED
digitilluminatestoindicatewhichtestisbeingper-
formed.Startingwiththenumeral7,itmakesthere-
quiredmeasurementandverifiesifitiswithintoler-
ancebyflashingthegreenPowerGuardlights.It
thencountsdowntosix,performsandverifiesthe
nexttest,thenfive,four,three,etc.untilitreaches
"1"andthenthespeakersturnon.A "beep"toneis
heardeachtimea testisperformed.Ifa circuit
shouldfail,bothred"limit"lightswillcomeonand
thesequencewillstopatthatpoint.Speakerturnon
willnotoccuruntilthefaulthasbeencorrected;
therebyprotectingyoursystemfromfurtherdam-
age.
Therearetwouserswitchesassociatedwiththe
autotestcircuit.Theyarelocatedontheamplifier
toppanel.Oneswitchcontrolsthespeedofthe
countdown(SLOWorFAST).Theothercontrolsthe
presenceofthebeeptone(ONorOFF).
-10-

RearPanelInformation
MODESWITCH
TheMC2250willoperateinthreemodes,Stereo,
MonoBridge,andMonoParallel.TheMonomodes
differintheloadstheywilldriveandtheconnection
sequencetothespeakerterminals.
LEFTandRIGHTOUTPUTTERMINALS
Forstereooperation,outputimpedancesof1,2,4
and8 ohmshavebeenprovidedonsecure,screw
typebarrierstripsforeachchannel.Formonophonic
operationproperinterconnectionprovides0.5,1,2,
4,8,and16ohmsfromthesamebarrierstrips.See
page4 forconnectinginstructions.
INPUTJACKS
Inthestereomodeofoperation,bothinputjacks
acceptsignal.Inthemonomodeofoperationonly
theRight(MONO)channelinputjackacceptssignal
andtheLeftchannelinputjackisdisconnected.
the2.5V positionandthefrontpanelLEFTand
RIGHT/MONOGAINcontrolsinthefullyclockwise
position.Ifmoregainisdesiredthe0.75V position
maybeused.Forsourceequipmentotherthan
Mclntosh,settheswitchinthepositionnearestto
thestatedoutputratingofthesourceequipment.
ACPOWER
TheinputtotheMC2250is120volts50/60Hzat
upto12amps.Theprimarycircuitisprotectedbya
15AmpFuse.
INPUTLEVEL
TheInputsensitivityoftheMC2250is0.75voltsor
2.5V dependingonthepositionoftheINPUTLEVEL
switch.AllMclntoshpreamplifiershavebeende-
signedtodeliver2.5voltsoutputwithratedinput.
ForthebestsignaltonoiseratiowhenusingMclntosh
sourceequipment,placetheINPUTLEVELswitchin
-11-

MONO
0.02%maximumharmonicdistortionatanypower
levelfrom250milliwattsto500wattsfrom20Hzto
20,000Hz.
INTERMODULATIONDISTORTION
STEREO
0.02%maximumifinstantaneouspeakpowerout-
putis500wattsorlessperchannelwithbothchan-
nelsoperatingforanycombinationoffrequencies,
20Hzto20,000Hz.
MONO
0.02%maximumifinstantaneouspeakpowerout-
putis1000wattsorlessforanycombinationoffre-
quences,20Hzto20,000Hz.
FREQUENCYRESPONSE(atonewattoutput)
20Hzto20,000Hz+ 0 - 0.25dB.
10Hzto100,000Hz+ 0.25- 1 dB.
NOISEANDHUM
95dBbelowratedoutput.
RATINGS
DAMPINGFACTOR
Greaterthan30
INPUTIMPEDANCE
50,000ohms.
INPUTSENSITIVITY
Switchable:0.75voltor2.5volts—levelcontrolpro-
videdforhigherinputvoltages.
POWERGUARD
ClippingispreventedandTHDdoesnotexceed2%
withupto20dBoverdriveat1 KHz.
GENERALINFORMATION
POWERREQUIREMENTS
120volts50/60Hz0.7to12amps.
SEMICONDUCTORCOMPLEMENT
76silicontransistors
37siliconrectifiersanddiodes
9integratedcircuits
MECHANICALINFORMATION
SIZE
Frontpanelmeasures16incheswide(40.7cm)by
6-31/32incheshigh(17.7cm).Chassismeasures
14-9/16incheswide(37cm)by6-1/2incheshigh(16.5
cm)by14-1/2inchesdeep(36.8cm),includingcon-
nectors.Knobclearancerequiredis1 inch(2.5cm)in
frontofmountingpanel
FINISH
Frontpanelisanodizedgold.Chassisischromeand
black
WEIGHT
80pounds(36.3kg)net,94pounds(42.6kg)inship-
pingcarton
-12-
PerformanceLimits
PERFORMANCEGUARANTEE
PerformanceLimitsarethemaximumdeviation
fromperfectionpermittedfora Mclntoshinstru-
ment.Wepromiseyouthatwhenyoupurchasea
newMC2250froma Mclntoshfranchiseddealer,it
willbecapableoforcanbemadecapableofperfor-
manceatorexceedingtheselimitsoryoucanreturn
theunitandgetyourmoneyback.Mclntoshisthe
onlymanufacturerthatmakesthisstatement.
PERFORMANCE
Mclntoshaudiopowerratingsareinaccordance
withtheFederalTradeCommissionRegulationof
November4,1974concerningpoweroutputclaims
foramplifiersusedinhomeentertainmentproducts.
POWEROUTPUT
STEREO
250wattsminimumsinewavecontinuousaverage
poweroutput,perchannel,bothchannelsoperating
into1 ohm,2 ohms,4 ohms,or8 ohmsloadim-
pedance,whichis:
15.8voltsRMSacross1 ohm
22.4voltsRMSacross2 ohms
31.6voltsRMSacross4 ohms
44.7voltsRMSacross8 ohms
MONO
500wattsminimumsinewavecontinuousaverage
poweroutputinto0.5ohm,1 ohm,2 ohms,4 ohms,8
ohms,or16ohmsloadimpedance,whichis:
15.8voltsRMSacross0.5ohm
22.4voltsRMSacross1 ohm
31.6voltsRMSacross2 ohms
44.7voltsRMSacross4 ohms
63.2voltsRMSacross8 ohms
89.4voltsRMSacross16ohms
OUTPUTLOADIMPEDANCE
STEREO
1ohm,2 ohms,4 ohms,and8 ohms;separateter-
minalsareprovidedforeachoutput.
MONO-PARALLEL
0.5ohm,1 ohm,2 ohms,and4 ohms;obtainedby
connectingtogethertheappropriateterminalsof
bothchannels.
MONO-BRIDGED
2ohms,4 ohms,8 ohms,or16ohms,balancedto
commonconnections.
RATEDPOWERBAND
20Hzto20,000Hz
TOTALHARMONICDISTORTION
STEREO
0.02%maximumharmonicdistortionatanypower
levelfrom250milliwattsto250wattsperchannel
from20Hzto20,000Hz,bothchannelsoperating.

PerformanceCharts
PERFORMANCECHARTSARE
FORONESTEREOCHANNEL
WITHBOTHCHANNELSOPERATING.
MONOOPERATIONISIDENTICAL
EXCEPTPOWERISDOUBLED
FORTHESINGLEMONOOUTPUT.
-13-

-14-

TechnicalDescription
INPUTAMPLIFIER
Eachchannelcontainsa completeseventransis-
torlowpowerinputamplifier.Aninputdifferential
transistorpairprovideshighinputimpedanceand
lownoise.Theoutputsofthispairarecombinedina
currentmirrorcircuitwhichdrivesa classA amplifi-
erstage.Theoutputstagefollowsthisamplifierand
isa complementarypairoftransistorsbiasedclass
AB.Theoutputsignaldrivesthemeteringcircuit,
headphonejack,andthehighpoweroutputamplifi-
er.Thisdiscretetransistoramplifierdesignwas
selectedforlownoise,lowdistortion,adequate
poweroutputcapabilitytodriveheadphones,and
freedomfromturnonandturnofftransients.
IntheMONOmodeofoperationtheinputsignal
feedsonlytherightinputamplifierviatheRIGHT/
MONOGAINcontrol.Therightinputamplifierfeeds
bothoutputpoweramplifiersections.WhenMONO
BRIDGEMODEisselectedtheleftchannelinputam-
plifierisusedasa phaseinverter.Theoutputchan-
nelsthenoperate180°outofphasewithrespectto
eachotherwhichisthecorrectarrangementfor
bridgeoutputconnections.WhentheMONOPAR-
ALLELMODEisselectedthechannelsoperatein
phase.Thechannels,ofcourse,alsooperatein
phasefortheSTEREOMODE.
OUTPUTPOWERAMPLIFIER
Theinputamplifiersfeedtheoutputamplifiers
throughanelectronicswitch.Theswitcheliminates
turn-onandturn-offtransientsandisusedforspeak-
erswitching.A junctionFETandLED/lightdepen-
dentresistornetworkmakeuptheswitch.Thiscom-
binationallowsthelowestpossibledistortionwhen
theamplifierisonandthehighestisolationwhen
theamplifierisoff.Thecontrolsignaltotheswitch
isoffduringtheautotestcountdown.Therefore,
transientsthatmaycomeintotheamplifierfrom
sourceequipmentwillnotbeamplifiedorreachthe
loudspeaker.SincetheHEADPHONEoutputand
metersarepoweredbytheinputamplifier,theirop-
erationisnotaffectedbytheSPEAKERswitchorthe
turnondelaysystem.
Thefirststageoftheoutputpoweramplifierisa
differentialtransistorpairbiasedforbestlinearity.
Theoffsettothedifferentialpairisadjustable.Cor-
rectadjustmentallowsthelowestpossibledistor-
tionatlowfrequencies.A currentmirrorcircuitcom-
binesthedifferentialoutputsintoonesignalwhich
isthenamplifiedbya followingclassA voltage
amplifier.Boththedifferentialtransitorsandthe
voltageamplifieraresuppliedbyactivecurrent
sources.Theresultsarelowerdistortionandcleaner
turn-oncharacteristics.
Thedriverstagefollowsthevoltageamplifier.The
driverconsistsofa complementarypairofpower
transitorsbiasedclassAB.Next,threecomplemen-
tarypairsofruggedpowertransistorsmakeupthe
poweroutputstage.Allpowertransistorsare
mountedonconservativelysizedanodized
aluminumheatsinks.Becauseofa uniqueconnec-
tionofthebiasnetwork,theoutputtransistors
operateclassB andexhibitnocrossoverdistortion
oftenassociatedwithclassB operation.Theheat
sinks,therefore,remaincoolwhenthereisnoout-
put.
Theamplifieroutputsignalisfedtotheoutputter-
minalsthroughtheoutputautotransformer.The
Mclntoshdesignedinterleavedmultifilarwound
autotransformerisusedtoproperlymatchthe
amplifiertostereooutputloadtapsfor1,2,4 and8
ohms.TheMC2250willdeliverfullpoweroverthe
entireaudiofrequencyrangeatanyoftheseim-
pedances.Theautotransformeralsoprotects
speakersfromdamageintheeventofamplifier
failure.Shoulda directcurrentcomponentappearin
theoutput,itisshuntedthroughthelowDC
resistanceoftheautotransformer.DCcannot
damagethespeaker.
AMclntoshpatentedSentryMonitoringcircuit
constantlymonitorstheoutputsignalandinstantly
reactstopreventoverloadoftheoutputtransistors.
Atsignallevelsuptoratedoutputthiscircuithas
highimpedanceandhasnoeffectupontheoutput.
Ifthepoweroutputexceedsdesignmaximum,the
SentryMonitoringcircuitoperatestolimitthesignal
totheoutputtransistors.Intheeventofa shortcir-
cuitacrosstheamplifieroutputorsevereim-
pedancemismatchtheSentryMonitoringcircuitwill
protecttheoutputtransistorsfromfailure.Both
positiveandnegativehalvesoftheoutputsignalare
monitoredandprotectedindependently.
POWERGUARDPROTECTIONCIRCUIT
TheMclntoshpatentedPowerGuardcircuit
eliminatesamplifierclippingduetooverdrive.The
circuitalsoilluminatesredLIMITindicatorlamps
whentheamplifierisdrivenbeyonditsmaximum
outputcapacity.PowerGuardpreventsloudspeaker
damageandeliminatesharshoutputdistortion
causedbyamplifierclipping.
ThePowerGuardcircuitconsistsofa waveform
comparatorwhichmonitorsthewaveshapeofthe
amplifierinputandoutputsignals.Normallythereis
-15-

nodifferenceinwaveshapebetweenthesesignals
andthecomparatorproducesnooutput.Whenthe
amplifierisdrivenbeyonditsmaximumpower
capacitya differencewilldevelope.Ifthisexceeds
0.5%(equivalentto0.5%totalharmonicdistortion)
thecomparatoroutputcausestheredLIMITin-
dicatorstolight.Ifthereisa furtherincreasethe
comparatoroutputactivatesanelectronicat-
tenuatorattheamplifierinputtoreducethe
amplifiergain,thisautomaticattenuatorwillhold
theamplifieroutputtoitsmaximumundistorted
valueregardlessofthedegreeofoverdrivetothe
amplifier.Theamplifiermaybeoverdrivenby20dB
beforetheoutputdistortionexceeds2%.
Thecomparatorisanespeciallycompensated
operationalamplifierintegratedcircuit.Itsoutputis
detectedbya fullwavebridgethatfeedssignalsto
thecontrolcircuitryfortheLIMITandNORMALin-
dicatorsandtotheelectronicattenuatoratthe
amplifierinput.Theattenuatorisa lightemitting
diode/lightdependentresistornetworkselected
especiallyforitslowdistortionandpropertimecon-
stantcharacteristics.
SYSTEMTESTCIRCUIT
WhenAClinepowerisappliedtotheMC2250the
SystemTestcircuitfunctionstoverifyoperating
potentialsat7 testlocationswithintheamplifier.
Uponapplyingpowera digitaldisplaylightswiththe
number7.Thedigitisdrivenbya counterthatsteps
downata 1 secondor.4secondrate.Eachtimethe
digitchanges,electronicswitchesselecttheout-
putsofvariousvoltagecomparatorsconnectedto
testpointsintheamplifier.Ifvoltagesarenormal
thegreenPowerGuardlightsflash,thereisanaudi-
blebeepproducedbyaninternalsounder,andthe
displaydecreasestothenexttestnumber.Whenthe
countpassesnumber1 theloudspeakersconnect
andthedisplayblanksoff.Ifa faultconditionexists
thecounterstopsandbothredPowerGuardlimit
lightsgoon.Thedigitdisplayedindicatesthecircuit
atfault.Servicingtheamplifieristherebysimplified.
Thetestcircuituseslogiccounter,BCDdecoder,
lampdriver,andvoltagecomparatorintegratedcir-
cuits.
POWERSUPPLY
Thepowersupplyisa conventionalfullwave
bridgerectifierarrangementprovidingplusand
minus50voltsDC.Electronicregulatorsstepdown
andregulateplusandminus15voltsourcesforlow
levelcircuitsandplus5 voltsforthelogiccircuits.
Thermistorsareusedinthepowertransformer
primarycircuittolimittheturn-oncurrent.
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MclNTOSHLABORATORYINC.
2CHAMBERSST.,BINGHAMTON,N.Y.13903-9990
607-723-3512
Thecontinuousimprovementofitsproductsisthepolicyof
MclntoshLaboratoryIncorporatedwhoreservetherightto
improvedesignwithoutnotice.
PrintedinU.S.A.
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